This study will enchance our understanding of the relationship between diet and breast cancer, especially as it relates to steroid hormone metabolism. The overall objective is to use a controlled feeding study to examine the effects of varying the type of fat, but not the amount of total fat, in the diet on steroid hormone metabolism in premenopausal women. In feeding study 1 (year 1), two diets, one high in saturated fat and the other high in polyunsatured fat, will be used. Another objective is to examine whether dietary soy may moderate the effect of dietary saturated fatty acids on hormone metabolism. In feeding study 2 (year 2), two diets, one high in saturated fat with no added soy and the other high in saturated fat with soy, will be used.